As known in the art of semiconductor fabrication, etch processes to form deep features (e.g., 1 μm, or more) are known to be susceptible to significant lateral undercut of the substrate under the masking layer(s). Many of these etch processes involve etching of the substrate material (e.g., Si) to form etched features, such as for trench isolation or through substrate vias (TSVs) (e.g., through silicon vias). The mask typically comprises photoresist, a dielectric, or a combination of the two (e.g., photoresist on a dielectric or a dielectric stack). Subsequent processes can involve filling of the etched feature. Significant lateral undercut of the substrate under the masking layer(s) can lead to a variety of fill problems, such as voids or improper plating in the case of TSV formation.
Some methods are known for reducing lateral mask overhang and as a result, the lateral substrate undercut. One method comprises complete removal of the sacrificial masking layer(s) after the etch, such as removal of a silicon oxide masking layer that is wet-etched off completely or a photoresist masking layer that is stripped off after the etch. Although this method is generally effective when the masking layer(s) are fully sacrificial, this method is not possible when at least a part (e.g., one or more layers of a mask stack) of the mask layer(s) are non-sacrificial and thus remain for subsequent processing. Another method comprises re-patterning with the original masking layer(s) that defines the overhang present after the etch, and then another etch step for etching the overhang portion of the masking layer(s) away. Moreover, this method requires an additional photolithography step that adds cost and process complexity due to generally complicated alignment/patterning.
Yet another method comprises deep etch (e.g., Si etch) process changes (different gas mixtures) to reduce lateral mask overhang to reduce the lateral substrate undercut. This method is often very costly in terms of loss of etch rate to achieve a reduction in lateral overhang. Moreover, it is generally difficult or impossible to largely eliminate the lateral overhang and resulting lateral undercut using this method, particularly for deep etch processes.